Magnetorheological finishing (MRF) is a polishing technology that uses a magnetorheological (MR) fluid. MRF was commercialized in 1997 by QED Technologies and is considered to be an excellent, deterministic process for finishing optics to high precision. A variety of CNC machines and MR fluids are used throughout the world on a regular basis.
MR fluids are the key element of MRF technology. In general, MR fluids consist of uniformly dispersed non-colloidal magnetic particles, e.g., carbonyl iron (CI) in a carrier fluid. Properties like plasticity, elasticity, and apparent viscosity change with the application of magnetic field. A typical MR fluid for MRF applications is compatible with most optical substrates, providing relatively high removal rates and acceptable smoothing for precision optics applications, without the risk of scratching the workpiece surface due to oversized abrasive particles, as may happen with a solid lap. Material removal is accomplished primarily by non-magnetic abrasive particles incorporated in the MR fluid.
The two current commercial MR fluid options contain either cerium oxide (CeO2) or nano-diamonds as non-magnetic polishing abrasives. The choice of non-magnetic abrasive material is determined by the physical properties (e.g., hardness) and chemical properties (e.g., chemical durability) of the workpiece to be finished.
MRF is a subaperture polishing process. For a conventional MRF setup, the MR fluid is pumped through a delivery system and ejected through a nozzle in the form of a ribbon onto a rotating vertical wheel. The ribbon stiffens upon passing into a region with a high magnetic field in the vicinity of the workpiece. The MRF removal function is characterized by a D-shaped polishing spot in the zone of contact between the ribbon and the workpiece, and the material removal rate is determined by the time of contact (e.g., dwell time), as well as other process and workpiece parameters. The temperature of the MR fluid is controlled by a chiller normally set to ˜20 degrees C.
The MRF removal function is very sensitive to the stability of the MR fluid. Changes in MR fluid properties can reduce the determinism of MRF over time (the nominal life time of a standard MR fluid is approximately two weeks compared to three to four months with a polyurethane polishing pad). Stabilizers, such as glycerol may be added to improve fluid stability (i.e., control viscosity and keep both magnetic and non-magnetic particles in suspension). However, for glass polishing, an excess amount of glycerol inhibits the water hydration at the workpiece surface which is needed for softening the glass surface.
Even though the MR fluid has only limited exposure to atmosphere, it can still absorb carbon dioxide, which lowers the pH of the fluid and contributes to the oxidation of CI. Corrosion may cause the MR fluid to change its compositional properties, which subsequently result in an unpredictable MRF removal function. Using deionized (DI) water as the carrier fluid provides only a limited solution to the problem. The use of buffers such as sodium carbonate (Na2CO3) increase the fluid pH to ˜10, resulting in a more stable fluid. Na2CO3 reduced the corrosion problem sufficiently to allow for the development of a commercial MR fluid for MRF.
Many coating and surface treatments have been applied to CI particles for use as MR fluids in industrial applications (e.g., vibration dampers, clutches, and actuating modules) to achieve benefits including improved sedimentation stability, improved dispersability, improved oxidation and corrosion resistance, and stability at higher solids concentrations. Coating media that have been explored include non-magnetic metals, ceramics, high performance thermoplastics, thermosetting polymers, polyvinyl butyral, polystyrene nano-spheres, silicon, phosphates, metal oxides like silica and zirconia, and combinations of some of the above. Enhancement of the particle surface with nitrogen has also been reported. Of the many coating application methods employed, the sol-gel method has often been used, because it is suited to a variety of materials and offers excellent process control.
For most optical finishing applications a water based MR fluid is used. As mentioned previously, the commercial MR fluids contain non-magnetic abrasive such as CeO2 and diamonds to enhance material removal and to control final surface roughness for a wide range of optical materials.
Water soluble crystals have important applications in optics. One example is potassium dihydrogen phosphate (KDP/KH2PO4), whose solubility is ˜21.7 g/100 g water at room temperature. KDP is the only nonlinear single crystal electro-optical material that can be grown in sizes large enough for use as a switch or as a frequency converter in solid state lasers used for investigating inertial fusion, such as the OMEGA and OMEGA EP at the Laboratory for Laser Energetics (LLE) of the University of Rochester, and the National Ignition Facility (NIF) at Lawrence Livermore National Laboratories (LLNL). It has been reported that a non-aqueous MR fluid composed of 40 vol. % CI, 0.05 vol. % nano-diamonds, and ˜60 vol. % dicarboxylic ester (DAE) as the carrier fluid could successfully polish a previously diamond turned KDP part to an rms surface roughness of ˜2 nm, removing all diamond turning marks.
Substituting the conventional non-magnetic abrasives in an MR fluid (i.e., CeO2 or nano-diamonds) with other commercial polishing abrasives may result in improved surface smoothing of relatively soft materials.
It has been reported that an MR fluid containing mechanically soft CI (˜4 μm diameter) and alumina abrasives could yield improved surface roughness for chemical vapor deposition (CVD) polycrystalline zinc sulfide (ZnS). This chemically altered MR fluid composition also showed no significant dependence on the initial surface preparation (single point diamond turning, pitch polishing, or deterministic microgrinding).
Zirconia (ZrO2) is a hard polishing abrasive used in conventional polishing of hard and soft glasses. Monoclinic zirconia is the preferred crystalline form for glass polishing, although cubic zirconia is also used. Excellent removal rates and surface roughness values have been reported for polymer [poly(arylene)ether] using 50 nm zirconia in comparison to ceria (CeO2), silicon oxide (SiO2), and tin oxide (SnO2). Fused silica (FS) polished with zirconia has been shown to leave surfaces that, upon laser damage testing in the UV and at 355 nm, exhibit superior damage resistance compared to surfaces polished with other abrasives. Applications for such surfaces exist in UV/DUV/EUV lithography for the semiconductor wafer industry, and in research laboratories exploring inertial confinement fusion. A polishing slurry consisting of a blend of zirconia and fumed silica was recently found optimal for chemical mechanical polishing (CMP) of a tetra-ethyl orthosilicate (TEOS) layer on a silicon wafer. The advantages of loose zirconia abrasives in conventional polishing have been reported in the literature.
The inventors have recognized a need for improved MR materials and processes for making such improved MR materials as well as the ability to manufacture these MR materials in batches of sufficient quantity for cost efficiency and usefulness. There also remains a need to provide improved corrosion resistance and life-stability for MR fluids, particularly for those used in the optical polishing field. It would be beneficial to provide novel MR materials and fluids having a variety of uses and applications.